Since RT 5.0 (I think) there is a support for sorting numerical customfields. But it’s not yet an easy configuration, you have to override RT::CustomField to mark the named CustomField as Numeric by returning true to the IsNumeric method: RT::CustomField - RT 6.0.2 Documentation - Best Practical
This is super useful! One extra thing to note - since RT 5.0.6 there’s built-in support for sorting custom fields numerically (so it won’t treat them like strings in search results) — but you do need to tell RT which CFs are numeric by overriding IsNumeric in a RT::CustomField_Local.pm and returning true for your field’s name. After that a restart should make the numeric sort work as expected.